Influence of graphite nodularity on microstructure and processing window of 1.5% Ni–0.3% Mo austempered cast iron

The effect of graphite nodularity on microstructure and processing window of austempered cast iron with four-different levels of graphite nodularity and a composition of 3.6% C, 2.2% Si, 0.2% Mn, 1.5% Ni, 0.3% Mo was studied. Microstructural observations and measurements of the retained austenite content and hardness, are reported as functions of austempering time at 440 and 300 °C after austenitising at 900 °C. The results of the experimental investigation indicate that graphite nodularity has considerable effect on the microstructure. After 8 min austempering time at 440 °C austempering temperature and 20 min austempering time at 300 °C austempering temperature, the amount of retained austenite (Xγ) increases with the graphite nodularity. The microstructural investigation and hardness test investigation indicate that the graphite nodularity has a significant effect on the time t1 corresponding to the end of stage I (process window), whereas decreasing the graphite nodularity delays the end of stage I reaction. The starting time of reaction II, t2 is determined from the variation of Xγ with austempering time, it is indicated that lower graphite nodularity accelerates the start of stage II reaction.